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ENERGY                                       193
nozzle is 15 feet above the surface of the reservoir which supplies the water.
11.   Find the power of a machine gun which projects 600 bullets per
minute with a muzzle velocity of 500 —- and angular velocity of 600 TT ra-
sec.
•dians per second.   The bullets are cylinders 0.9 cm. in diameter and 15 gm. mass.
12.   A shaft transmits 50 horse power and makes 150 revolutions per minute.   Express the torque transmitted in pounds-foot and dynes-cm.
13.   An electric motor develops 25 kilowatts at 900 revolutions per minute.   Find the torque on the rotating armature due to the field magnets.   Neglect friction.
14.   Find the power of a clock which has a maximum run of 8 days. 'The weight which moves the works has a mass of 10 kg.   At its highest position the weight is 15 inches above its lowest position.
16. A twin-screw steamer has engines of 20,000 horse power and when working at full power the engines make 75 revolutions per minute. Find the torque transmitted by the shaft of each screw.
16. The pitch of the screw propeller of a ship is 25 feet. The power transformed by the propeller is 15,000 horse power, when the ship makes 20 knots. Assuming that there is a slip of 10 per cent at the propeller .screw and that the efficiency is 0.75, find the torque transmitted by the shaft, also the thrust on the bearings.
17. A feed pump delivers water into a boiler at the rate of 20 Ibs. an hour. If the pressure in the boiler is 150 Ibs. per square inch above the .atmospheric pressure, find the effective horse power of the pump.
POTENTIAL ENERGY.
158.   Configuration. — The arrangement of the parts of a •system is called the configuration of the system.    The system which consists of this book and the earth, for instance, is in one configuration when the book is on the desk and in another configuration when it is on the floor.    During the transfer of the book from the floor to the desk the system passes, continuously, through infinite number of configurations, because the book occupies infinite number of different positions relative to the earth.
159.   Conservative Forces. — If the work done in bringing £, system from one configuration to another configuration is